Art is one of the toughest hurdles to publishing a video game. Commissioning artwork actually involves a lot more than just contacting an artist and asking them to produce some 2D images. Essentially the developer becomes a mini-art director, providing guidance on how to compose the images--content, theme, feel, dimensions, fit with the surrounding images within the game... it can get pretty complicated. For more information, check out The Art Order, an awesome blog run by Jon Schindehette, who does real fantasy art direction for Dungeons & Dragons over at WotC.
Anyway, cost tends to be a prohibitive factor for low-budget video games. Recently I stumbled across the Publisher Resources section on Drive Thru RPG. Bingo! You can obtain high-quality art for fantasy, sci fi, steampunk and other genres. Most importantly, it's cheap. Granted, the art has probably been used in many products, but if you're trying to decide between a $500 commission and a $5 clipart painting, it's pretty obvious which one to choose. I dropped about $50 and got 100-200 images of characters, equipment, scenes and other material. Simply awesome. I highly recommend clipart on Drive Thru RPG to fill out your art content, especially if your development budget is very sensitive to cost.